In 2004, he protested the U.S.-led war in Iraq with a rap song calling for the killing of "Yankees who have been torturing Iraqi captives." The lyrics included: "Kill their daughters, mothers, daughters-in-law, and fathers. Kill them all slowly and painfully."

In his apology Friday, the 34-year-old singer said that performance reflected a "deeply emotional reaction" to the Iraq War.

"I will forever be sorry for any pain I have caused by those words," he said, adding: "While I'm grateful for the freedom to express one's self, I've learned there are limits to what language is appropriate and I'm deeply sorry for how these lyrics could be interpreted."

Psy is one of several performers scheduled for the "Christmas in Washington" concert, now in its 31st year; others include Diana Ross, Demi Lovato and Scotty McCreery.

Comedian and talk show host Conan O'Brien hosts the special at the National Building Museum, which will be taped Sunday night and aired Dec. 21 on TNT.